Scope of the IRP Assessment

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irradiation experiment at Oak Ridge’s reactor on steel samples would take approximately 10 years.

Future nuclear energy systems will require structural materials and fuels that can operate in environments that are more aggressive and experience higher levels of radiation damage.

Depending on the reactor concept the lifetime radiation damage requirements of in-core materials is between 10 and 200 dpa [86].

Many future power reactor designs have a much ‘harder’ neutron spectrum than today’s moderated reactors. The fast neutron displacement rate for structural materials is much higher so new materials need to be developed. There are few fast irradiation facilities, e.g. the BOR-60 reactor, capable of testing such materials.

Similar problems exist with testing materials for future fusion reactors. The IFMIF DONES project (International Fusion Materials Irradiation Facility DEMO Oriented Neutron Source) aims at building a neutron radiation source of energy and fluence similar to DEMO, to study and select the materials that will be needed for that project and in future fusion power plants [87].

The facility aims to provide 30‒40 dpa rapidly for accelerated testing of candidate materials.

Material irradiation testing often requires very large scale facilities. The International Fusion Materials Irradiation Facility (IFMIF, currently at the design stage) has been proposed for fusion reactor environment studies providing appropriate neutron energy and flux. The facility will consist of two 125 mA deuteron accelerators, a liquid Li target, and an irradiation test facility. Due to its scale, construction costs are expected to be 1.4 billion Euros, half-sized facilities that use one accelerator, including A-FNS and IFMIF-DONES neutron source programs [88], are currently proposed in Japan and the EU, respectively, for operation in 2030.

These material irradiation test facilities can supply fast neutron fluxes sufficient to achieve 20 dpa per year, but the test volume is relatively small compared to the test volume of nuclear reactors. So, small specimen and test methods will have to be developed and applied and may need to be discussed with the regulatory body. Moreover, heavy ion irradiation facilities and fission research reactors can be used to support the development of fusion materials. Smaller CANS are being developed for fast neutron damage (e.g., the cyclotron facility at Řež described in Section 4.1.2.).

Steady state compact fusion neutron sources have been proposed in several options, for material irradiation tests, integral effect tests of fusion reactors, and especially for the breeding blanket development.

3.1.2. Radiation hardness of electronics

3.1.2.1. Hardware errors

Radiation effects on electronics is a very important field of scientific and technological research. Radiation tolerance is of prime importance for many applications of electronics: space applications, high energy physics, nuclear reactors, and nuclear medicine. Showers of cosmic neutron radiation are intense enough that they may disrupt the normal operation of electronic systems and represent a particular threat to aircraft avionics.

3.1.2.2. Software errors

Neutron induced Single Event Effects on electronics may result in recoverable soft errors such as a Single Event Upset, e.g., temporary memory data corruption or logic circuit status changes, as well as a permanent hard error. The main material used in semiconductors is Si, and Si has three naturally occurring isotopes: ²⁸Si (92.23%), ²⁹Si (4.67%), and ³⁰Si (3.1%). Soft errors are caused by charged particles produced by neutron reactions mainly with Si nuclei. For fast neutrons, spallation reactions occur and produce charged particles. On the other hand, for lower energy neutrons, the major reactions are Si(n,p) and Si(n,α) reactions (See Table 2). The threshold energies indicate that neutron energy above about 10 MeV is necessary for an effective acceleration test of the devices although the small soft error cross section remains low below a few MeV as shown in Ref. [89]. Some components contain boron, and the capture of thermal neutrons by ¹⁰B is also a significant hazard that is receiving attention.

A list of facilities in the USA offering accelerators for Single Event Effects testing of space applications using protons or fast neutrons is given in Appendix B of Ref. [85]. Some of the larger neutron sources such as the ISIS spallation source, UK, offer Single Event Effects testing with a near atmospheric spectrum of neutrons for avionics with dedicated facilities such as ChipIR [90]. Some accelerators with lower energy proton beams such as NEPIR, a 30‒70 MeV cyclotron in Italy, are developing facilities such as ANEM (Atmospheric Neutron Emulator) for this purpose [91].

Tests using a CANS based around an electron linac at Hokkaido University, HUNS, whose source intensity is ~10¹² n/s at the target with an evaporation spectrum with high energy neutrons over about 10 MeV have been performed to test components at an accelerated rate.

An error rate of about 1 million times larger than natural was observed at this CANS [92]. An international standard of soft error testing for telecommunication systems was approved at ITU-T (International ITU-Telecommunication Union ITU-Telecommunication Standardization Sector) in 2018, providing a guideline for use of CANS for such purposes [93].

TABLE 2. NEUTRON REACTIONS WITH Si

Reactions with Si Threshold energy

28Si + n →²⁸Al + p 3.999 MeV

28Si + n →²⁵Mg +  2.749 MeV

29Si + n →²⁹Al + p 3.009 MeV

29Si + n →²⁶Mg +  35 keV

30Si + n →³⁰Al + p 8.040 MeV

30Si + n →²⁷Mg +  4.341 MeV 3.1.3. Radiobiology

Neutron sources are required in order to provide experimental data on radiation damage to human and animal tissues. Data on ‘non-traditional’ neutron fields (i.e. those not typical of fission) are required.

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3.1.3.1. Radioprotection of humans

Ionizing radiation is present in a variety of environments and workplaces, e.g., at nuclear reactors, radiotherapy facilities, aircrafts at high altitude, and outer space. In all cases, there are undesired or natural radiation fields. In order to better assess the health effects and risks on the general public, nuclear energy workers, health system patients and workers, pilots, flight attendants, and astronauts, it is important to have the possibility to perform experimental studies on the impact of radiation on living organs, tissues and cells (e.g. chromosome aberrations in human blood lymphocytes) [94]. The biological effectiveness of radiation depends on its type (electron, proton, alpha, neutron, gamma, ion), its energy, its distribution in time and space, and on the characteristics of the biological target. Radioprotection protocols, which consider safety margins, make use of a dose equivalent (in Sieverts) taking into account the radiation type and energy. In radiation therapies a relative biological effectiveness factor that considers radiation type, energy and target tissue is typically used. There are also more advanced methodologies that try to make a more realistic bridge between radiation energy deposition and overall biological and clinical effects. All high energy particles interact with matter by creating various other secondary particles. As the cross sections of all these reactions are energy dependent, it would be advantageous to be able to modify the spectrum of the incoming neutron or gamma beams to mimic the true spectrum that humans are exposed to; e.g., astronauts at the International Space Station [95]. Some of the higher powered CANS may contribute to this area.

The advent of accelerator based BNCT is demanding more radiation biology studies to support its development (see Section 3.5).

3.1.3.2. Induced mutagenesis

Induced mutagenesis is an indispensable tool for the creation of new alleles which can be explored for crop improvement. It has great importance, particularly where natural sources for the genetic variations are limited. By means of inducing mutation using different mutagenesis approaches, evolution can be accelerated or directed to achieve the desired change in an organism. In plant breeding programs, physical and chemical mutagens are applied to develop new varieties with enhanced traits. Crop mutation breeding using gamma rays is a long-standing technique. Neutron radiation is a relatively new approach to induce mutagenesis in seeds for crop improvement [96]. CANS can provide a fast neutron spectrum for that purpose.